Regress: Use --schema= option to change were the extension is installed

[Algunenano]

Currently the "--schema" option is used to check functions that might access objects or relations in the extension. We should extend it and use it also to install the extension into that schema.

[Algunenano]

The option was added in r17954

[strk]

In 17971:

[run_test] Use --schema as target for creating and finding postgis

Closes #4551

[Algunenano]

Related to this: We should change the files that we load for regress testing and, instead of reading the ones with @extschema@ stripped, we should replace @extschema@ with whatever schema is passed.

That way we can test pgplsql functions with something like:

BEGIN;
SET search_path = pg_catalog;
-- Query with the function
ROLLBACK;

in order to check that is independent on the search_path (e.g. everything is qualified properly)

[strk]

Let's go there incrementally, or file a ticket if you prefer